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[Phys-L] Re: sound frequency in helium



Julie,

It is only sort of a change in pitch.

Suppose you sing a particular note - i.e. produce a particular fundam=
ental frequency for the vibrations of the vocal cords. Of course, th=
ere are also harmonics of this frequency that are produced. The part=
icular sound that comes out depends on the shape of the mouth/tongue/=
throat. The size and shape of the vocal tract creates different res=
onances which enhance or repress different harmonics. The relative a=
mount of different harmonics determines determines the quality (or ti=
mbre) of the sound.

When you have a mouth full of helium, the speed of sound is much high=
er. Even if the vocal tract is the same shape, the resonace frequenc=
es are much higher. The lowest frequencies are no longer in resonanc=
e, so they are attenuated. New, higher pitched harmonics are in reso=
nance, so they are enhanced.

In a nutshell - the original vibrations are the same, but the resonan=
t frequencies are much higher.

Tim F


-----Original Message-----
=46rom: Forum for Physics Educators on behalf of Julie Quah
Sent: Sun 4/17/2005 8:56 PM
To: PHYS-L@LISTS.NAU.EDU
Subject: sound frequency in helium
=20
Dear experts, pleasse help me out with this issue of sound pitch in h=
=3D
elium=3D20
gas.
When one breath helium gas, and speak, it is found that the pitch inc=
=3D
reases.=3D20
How do we explain this? But if we compare the water wave travelling t=
=3D
hrough=3D20
deep to shallow water, its speed changes without frequency chage, as =
=3D
the=3D20
source (motor frequncy) is the same.
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